It is said, and rightly so, that cosmology is the branch of physics that asks the grandest questions. After all, few questions within science can equal the impact of: “Where does the universe come from?” or “What is the fate of the universe” or “Where does the matter we are made of come from?”
But perhaps even more exciting than asking these questions is the fairly recent power that we have of answering them, at least partially, through a rational study of nature.
Most of us learned in high school that matter is made of atoms and that atoms are made of protons, neutrons and electrons. What we don’t usually learn in high school is that to each particle of matter there is another particle, an “anti-particle,” which is essentially the same as the particle but with opposite electric charge.
Thus, the negatively charged electron has its “anti-electron,” called a positron, which has positive electric charge; the proton has an anti-proton, and so on. Now comes the interesting part. According to the laws of particle physics, matter and antimatter should be present in the universe in equal amounts. And yet, we have ample observational evidence that, at least in a very large volume that surrounds us and extends far beyond our galaxy, there is much more matter than antimatter.
When particles collide with their anti-particles, the effects are devastating; they both disintegrate into electromagnetic radiation, their energy carried away in neutral particles called photons. In other words, if there were as much antimatter as matter in the universe, we wouldn’t be here to ask grand questions. The universe is somehow unbalanced, biased toward the existence of matter over antimatter. One of the greatest challenges in modern cosmology is to unveil the roots of this cosmic imperfection.
As with any scientific explanation, we need a few “basic ingredients,” a minimum amount of knowledge from which to build our models. The first ingredient we need is the Big Bang model of cosmology. According to this model, a small fraction of a second after the “beginning,” many kinds of particles and their anti-particles, in equal amounts, roamed about and collided with each other immersed in tremendous heat, as in a cosmic minestrone soup.
In this hot cosmic furnace, many different types of particles were being cooked, not necessarily the familiar quarks (the constituents of protons and neutrons) or electrons. As the universe expanded and cooled, a sort of selection mechanism not only biased the creation of quarks and electrons over other types of particles, but also generated the excess number of particles over anti-particles.
Surviving the annihilation with their antimatter cousins, these excess particles organized themselves into more complex structures, until eventually atoms, mostly hydrogen, were formed when the universe was about 300,000 years old. The mystery, then, is to understand what kind of physics could generate this bias.
At first, resolving this question seems impossible. How can we possibly understand the mechanism that selected the existence of matter over antimatter during the earliest stages of evolution of the universe? In 1968, Andrei Sakharov, best known as the father of the Soviet bomb, proposed a recipe to generate more matter than antimatter in an expanding universe.
He suggested that three conditions must be satisfied in order to produce the matter excess. First, there must be a way of creating both more matter and antimatter particles of the kinds which are important to us—that is, the kinds that make up the atoms we are made of. Then, there must be a mechanism to bias the creation of more matter than antimatter. And finally, once we have an excess of matter particles over their antimatter partners, we must make sure that this excess is not erased as the universe continues to expand.
The first of these conditions is the creation of both baryons and anti-baryons from collisions involving the other particles present in the primordial soup. Baryons are particles which interact via the strong nuclear force, the force responsible for holding the nucleus together. Protons and neutrons (a.k.a. nucleons), and their constituent parts called quarks, are all baryons. At low energies, the number of baryons participating in collisions between different particles is conserved: that is, just like electric charge, the total number of baryons before an interaction equals the total after. If we are interested in making baryons, as we must in order to create matter in the universe, this conservation law is not very useful. According to Sakharov’s requirement, however, at very high energies the interactions between elementary particles should not conserve the number of baryons. That is, at high energies both baryons and anti-baryons can be created from “other” particles. These high energies are naturally realized in the hot furnace of the early universe.
But this first condition does not differentiate between baryons and anti-baryons. At high temperatures we could still create the same number of each, and that wouldn’t cause a bias toward matter over antimatter. We need a second condition. Once the high energies of the early universe allow for the creation of baryons and anti-baryons, we need a condition that selects, or biases, the creation of baryons over anti-baryons, an arrow pointing in the correct direction (i.e., toward matter).
In 1964, J.H. Christenson and his collaborators found experimental evidence that interactions between certain baryons do indeed exhibit this bias.
It is as if Nature has its own biases, in this case toward more baryons. If this is true in laboratory experiments, no doubt this will also be true in the early universe. Making excess matter over antimatter is not as hard as it initially seemed to be. But this is still not the whole story. One more challenge remains, which has to do with the physics of hot systems, also known as thermodynamics.
One of the properties of very hot systems is that they have no memory of their past. Imagine a coffee spoon which is initially cold. Now immerse one of its ends into a very hot cup of coffee. What happens? Although initially only the end in the coffee will be hot, very quickly the whole spoon will be equally hot. You won’t be able to tell which of the two ends was immersed into the coffee cup; the system (coffee spoon and hot coffee) lost its “memory.” Another term for this loss of memory is thermal equilibrium. If the early universe was in thermal equilibrium, any excess baryons would have been deleted; in equilibrium, the net baryon number is zero. In order to maintain the baryon bias as the universe cools, we need to make sure the universe doesn’t “lose its memory” and delete the new baryons. Therefore, we need a third condition.
We need what are called “out of equilibrium” conditions. In order to “freeze” the net number of baryons produced by the first two conditions, the early universe could not have been always in thermal equilibrium. We are very familiar with systems that are out of thermal equilibrium in our everyday life. An example is condensation of steam. More specifically, imagine a container filled with hot steam which is immersed into a large bucket with cold water. The steam, being too hot compared with the cold water, is out of thermal equilibrium. In order to attain equilibrium it will go through a phase transition; the steam will cool down and condense, going from a gas phase to a liquid phase. As it does so, we will observe the appearance of droplets of the liquid phase that will grow and coalesce. The phase transition ends when the steam is completely converted into water.
How does this reasoning apply to the early universe? Strange as this may sound, the universe also went through phase transitions. Particles—and their properties—are also sensitive to temperature. The standard model of particle physics successfully describes how particles interact at energies over a thousand times larger than nuclear energies. According to this model, at very high temperatures all particles but one, the so-called Higgs particle, have no mass, while at lower temperatures they acquire a mass through their interactions with the Higgs particle. We say that matter has two different “phases,” above and below the temperature at which particles like the quarks and the electron acquire a mass.
Thus, as the temperature of the early universe dropped, it went through a phase transition, and particles gained their mass. Like water droplets in steam, droplets of the low temperature (massive) phase appeared within the high temperature (massless) phase, growing and coalescing, in a typical out-of-equilibrium phase transition. Since only in the high temperature phase are baryons created in excess over anti-baryons (recall that the first two conditions apply only at high temperatures), these excess baryonic particles will penetrate the droplets of the massive phase, like viruses invading cells, becoming the net baryon number in the low temperature phase. As the droplets grow and coalesce, the whole universe is converted into the massive phase, completing the phase transition. According to our current models of “baryogenesis,” the creation of the excess baryons occurred when the universe was about one thousandth of a billionth of a second old. The protons and neutrons we are made of are the fossils of this primordial event.
So is this it? Is our work finished? Far from it. The simplest particle physics models we have do not generate the observed excess of matter over antimatter. Even worse, our true understanding of the complicated dynamics of these phase transitions is at best incomplete, leaving many questions unanswered at the moment. We have the broad outline of an explanation for the generation of matter in the universe, but the details are far from being understood.